This invention relates to magnetic tunnel junction (MTJ) devices and methods for fabrication of MTJ devices having properties of reduced noise, electrical resistance, increased magnetoresistance, and increased magnetic field sensitivity.
The discovery of large magnetoresistance in magnetic tunnel junction devices (MTJs) at room temperature has renewed intensive interest in this type of device. In part, this interest is due to the potential applications in sensitive magnetic sensors and in non-volatile magnetic random access memory (MRAM). The key component in an MTJ device is a sandwich structure (metal/insulator/metal) consisting of two ferromagnetic (FM) metallic layers (top and bottom electrodes) separated by a thin insulating barrier. The barrier is thin enough to allow quantum mechanic tunneling to occur between two ferromagnetic layers. The tunneling resistance of MTJ device depends on the relative orientation of the magnetization vectors (M) in the two FM layers. The magneto-tunneling effect exploits the asymmetry in the density of states of the majority and minority energy bands in a ferromagnet. The larger the asymmetry the larger the spin polarization is, and so the larger the magneto-tunneling effect.
When subject to an external magnetic field, an MTJ device suffers a change in its electrical resistance. The relative resistance change is called magnetoresistance (MR) or the MR ratio, defined as:                                           Δ            ⁢                          xe2x80x83                        ⁢            R                    R                =                                            R              ⁡                              (                H                )                                      -                          R              S                                            R            S                                              (        1        )            
where R(H) and Rs are resistance values, at a measurement magnetic field H, and at saturation field, respectively. Beyond the saturation field, resistance remains at a constant value of Rs. The property of MR as defined in relation (1) has been used to sense magnetic field by measuring resistance change in a field. In general, a good magnetoresistive sensor is characterized by a large MR value achieved at a small saturation field. To obtain a large MR ratio, the quality of the tunnel barrier is critically important. The thin insulating barrier should be smooth, pin-hole free, well oxidized, and of proper stoichiometry.
In MTJ devices, when the M vectors are parallel in the two FM electrodes, there is a maximum match between the numbers of occupied electron states in one electrode and available states in the other. The electron tunneling current is at maximum and the tunneling resistance (R) minimum. On the other hand, in the antiparallel configuration, the electron tunneling is between the majority electron states in one electrode and minority states in the other. This mismatch results in a minimum current and a maximum resistance. In a typical MTJ sensor, the M vector of one FM electrode is pinned by an adjacent antiferromagnetic layer via so called xe2x80x9cexchange biasxe2x80x9d coupling effect. The M vector of the other FM electrode is free to rotate. Since an external field can easily alter the direction of this M vector, the tunneling resistance is sensitive to the field to be measured. According to Julliere""s magnetotunneling model, xe2x80x9cTunneling between ferromagnetic filmsxe2x80x9d, Physics Letters, vol. 54A, No.3 (1975), pp.225-226, the maximum MR ratio between parallel and antiparallel configurations is                                                         Δ              ⁢                              xe2x80x83                            ⁢              R                        R                    =                                                                      R                                      ↑                    ↓                                                  -                                  R                                      ↑                    ↑                                                                              R                                  ↑                  ↑                                                      =                                          2                ⁢                                  P                  1                                ⁢                                  P                  2                                                            1                -                                                      P                    1                                    ⁢                                      P                    2                                                                                      ,                            (        2        )            
where P1 and P2 are the spin-polarization factors of the two electrodes. For a transition ferromagnetic metal (Co, Fe, Ni, and their alloys), P is in the range of 20-40%, leading to xcex94R/Rxcx9c8-38%. For half-metals with a full spin polarization (Pxcx9c100%), the MR ratio can theoretically approach infinity, which is the characteristic of a perfect spin valve.
MTJs offer a set of major advantages as spintronic devices over other magnetic devices such as devices based on anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR). Some of the advantages include, but are not limited to, the following.
The junction resistance (R) of an MTJ can be varied easily over a wide range (10xe2x88x922-108 xcexa9), while keeping the large MR ratio intact. The value of R depends on barrier thickness (txcx9c0.5-2 nm) exponentially and on junction area (A) inversely. The ability to tailor R in MTJ to suit electronics surpasses that in GMR devices.
MTJ can be miniaturized to nanometer size while retaining an adequate resistance, because R is primarily sensitive to barrier thickness. This property, not available in GMR spin-valves, is particularly important for high-resolution field imaging.
MTJ devices can operate in a very large frequency range (0-5 GHz) with good response.
MTJ devices are simple two-terminal resistive devices, requiring only small current density to operate. The stray field generated by the sensing current is small.
MTJ devices have a larger MR ratio. For example, a MR value as high as 49.7% at room temperature has been reported in MTJs with electrodes composed of Co75Fe25, an alloy with a high spin polarization. In contrast, a commercial (Fexe2x80x94Ni)/Cu/Co GMR sensor has a maximum MR of 9%.
One other major advantage of MTJ devices is that the magnetic coupling between the sensing layer and the pinned layer is weak because of the absence of RKKY magnetic interaction that is found in GMR sensors.
When characterizing an MR sensor, many researchers would use the MR ratio as a figure of merit. However, for field-sensing applications, a large MR ratio alone is insufficient. It is the intrinsic noise figure, both magnetic and electric, that determines the ultimate sensor performance. While reductions in noise are critical, and external noise reductions are relatively simple to achieve, control of a sensor""s internal noise is more difficult. Failure to adequately reduce the sensor""s internal noise could impede or swamp detection of small field modulations, regardless of the MR ratio. The field-sensing ability of the MTJ can be complicated by many internal noise sources: Johnson-Nyquiist (limited by resistance and temperature), tunneling current (shot noise), 1/f (two-level systems from defects), Barkhausen (domain-wall movement), and thermal fluctuations in magnetization. For typical sensing and memory applications, it is paramount that the magnetic and electric noise of an MTJ device be reduced as much as possible.
Prior to the present invention disclosed herein, there has been no effort to develop fabrication and post-deposition processes to reduce the noise in MTJ devices. S. Ingvarsson et al., measured the electric and magnetic noise in non-optimized MTJ memory devices but did not include sensor devices. Results were presented in xe2x80x9cElectronic noise in magnetic tunnel junctionsxe2x80x9d, Journal of Applied Physics, vol. 85, page 5270 (1999) and in xe2x80x9cLow frequency magnetic noise in magnetic tunneling junctionsxe2x80x9d, Physical Review Letter, vol. 85, page 3289 (2000). E. R. Nowak, et al., measured the electronic noise in non-optimized MTJ memory (not sensor) devices, but did not evaluate the magnetic noise, as presented in xe2x80x9cNoise properties of ferromagnetic tunnel junctionsxe2x80x9d, Journal of Applied Physics, vol. 84, page 6195 (1998) and in xe2x80x9cElectric noise in hysteretic ferromagnet-insulator-ferromagnet tunnel junctionsxe2x80x9d Applied Physics Letter vol. 74, page 600 (1999). In another electronic noise study, it was claimed that no magnetic noise was observed in the MTJ samples. This study was published by Daniel S. Reed in NVE, in xe2x80x9cLow Frequency Noise in magnetic Tunnel Junctionsxe2x80x9d, IEEE Transactions on Magnetics, vol. 37, page 2028(2001). However, this invention shows that magnetic noise definitely exists in MTJ devices, and represents the dominant source of noise.
Even though MTJ devices have larger MR ratios than AMR or GMR devices, no effort has been made so far to reduce the intrinsic noise of MTJ devices. However, in both sensing and memory applications, low noise levels are a requirement. Various improvements in sensing and memory applications are thus contingent upon the development of improved sensing devices.
The foregoing and other problems are addressed and solved by the teachings in accordance with this invention.
Disclosed herein are low noise, low resistance, high sensitivity, and large magnetoresistance magnetic tunnel junction (MTJ) devices, and methods for fabricating these devices.
The devices produced by the method disclosed herein offer significant improvements in magnetoresistance, resistance, field sensitivity, and noise level over existing devices, as confirmed by structural, magnetic, and transport characterizations. For example, observations have revealed these devices are capable of antiferromagnetic/ferromagnetic interfacial exchange bias fields of 420 Oe and magnetoresistance of up to 38.0%. Linear and non-hysteretic field sensing response has been achieved by providing a moderate hard-axis bias field. Under optimal thermal annealing, intrinsic magnetic noise was reduced to only 1 nT/Hz1/2, which is about 0.0002 of the earth""s magnetic field. Simultaneously, the magnetoresistance and sensitivity are highest at 35% and 5%/Oe.
The MTJ devices are fabricated through a multi-step process. In this process, a series of seven layers are deposited upon a substrate. In a preferred embodiment, the substrate is formed of silicon, and the surface is prepared with thermally oxidized SiO2. Once the substrate has been prepared, the layers are sequentially deposited with the first application being called the buffer layer. Subsequent to the buffer layer, a seed layer is applied, followed by an antiferromagnetic (AFM) layer, a pinned layer, a barrier layer, a free layer, and last of all a passivation layer.
In the preferred embodiment, the layers are deposited sequentially in the order of a thickness of about 30 nm of Pt (as the buffer layer), a thickness of about 3 nm of Ni81Fe19 (seed layer), a thickness of about 13 nm of Fe50Mn50 (AFM layer), a thickness of about 6 nm of Ni81Fe19 (pinned layer), a thickness of about 0.5-2 nm of Al2O3 (barrier layer), a thickness of about 12 nm of Ni81Fe19 (free layer), and a thickness of about 49 nm of Al (passivation layer). These thicknesses, or materials, however, should not be interpreted as limitations upon the practice of this invention.
In the preferred embodiment, the layers are deposited through a process of sputtering with a DC magnetron in a reduced pressure environment while a magnetic field is applied to induce uniaxial anisotropy in the ferromagnetic layers. Deposition occurs at room temperature. The Al2O3 layer is formed by oxidizing a thin layer of Al at an increased pressure for a specific period of time. Once the Al2O3 has been deposited, the pressure is again reduced and the residual oxygen gas is substantially removed from the fabrication area. These steps occur prior to the deposition of the Ni81Fe19 in the free layer. Once the Ni81Fe19 has been sputtered over the Al2O3 layer, an Al passivation layer is deposited to protect the structure from oxidation.
After the process of depositing the layers on the silicon wafer has been completed, the structure is divided into a plurality of appropriately sized segments. In the preferred embodiment, lithography techniques are used to divide the structure into segments appropriately sized for use as MTJ devices. Electrical contacts may conveniently be implanted into the segments during this step of fabrication.
Finally, the segments are annealed under conditions that have been set to optimize the performance of important properties of the MTJ. For instance, optimal temperature for annealing is determined through correlating the MTJ device performance to results from a series of annealings completed at varied temperatures. In this manner, selection of optimal annealing conditions for maximizing performance of an MTJ device is determined.
In the preferred embodiment, the MTJ devices are annealed for approximately ten minutes at a temperature of approximately 168xc2x0 C to 170xc2x0 C.